Recent technological advances have led to complementary metal-oxide-semiconductor (CMOS) sensor imagers being leveraged by cameras, video systems, and the like. CMOS sensor imagers can include an integrated circuit with an array of pixel sensors, each of which can comprise a photodetector. Moreover, a CMOS sensor imager can be incorporated into a System-on-Chip (SoC). As such, the SoC can integrate various components (e.g., analog, digital, . . . ) associated with imaging into a common integrated circuit. For instance, the SoC can include a microprocessor, microcontroller, or digital signal processor (DSP) core, memory, analog interfaces (e.g., analog to digital converters, digital to analog converters), and so forth.
Visible imaging systems implemented using CMOS imaging sensors can reduce costs, power consumption, and noise while improving resolution. For instance, cameras can use CMOS imaging System-on-Chip (iSoC) sensors that efficiently marry low-noise image detection and signal processing with multiple supporting blocks that can provide timing control, clock drivers, reference voltages, analog to digital conversion, digital to analog conversion and key signal processing elements. High-performance video cameras can thereby be assembled using a single CMOS integrated circuit supported by few components including a lens and a battery, for instance. Accordingly, by leveraging iSoC sensors, camera size can be decreased and battery life can be increased. Also, dual-use cameras have emerged that can employ iSoC sensors to alternately produce high-resolution still images or high definition (HD) video.
A CMOS imaging sensor can include an array of pixel cells, where each pixel cell in the array can include a photodetector (e.g., photogate, photoconductor, photodiode, . . . ) that overlays a substrate for yielding a photo-generated charge. A readout circuit can be provided for each pixel cell and can include at least a source follower transistor. The pixel cell can also include a floating diffusion region connected to a gate of the source follower transistor. Accordingly, charge generated by the photodetector can be sent to the floating diffusion region. Further, the imaging sensor can include a transistor for transferring charge from the photodetector to the floating diffusion region and another transistor for resetting the floating diffusion region to a predetermined charge level prior to charge transference.
CMOS imaging sensors can exhibit fixed pattern noise. Pixels within the array can yield signals associated with differing intensities relative to general background noise. Further, the fixed pattern noise can cause each pixel in the array to present a similar noise pattern in images captured over time while under similar conditions (e.g., temperature, exposure, . . . ); for instance, a pixel with high intensity due to the fixed pattern noise captured in an image at a first time can exhibit similar high intensity due to the fixed pattern noise at a second time. Inclusion of the fixed pattern noise can therefore distort images yielded by CMOS imaging sensors. Conventional techniques mitigate fixed pattern noise on a column by column basis (e.g., reduce column fixed pattern noise); however, such techniques can yield images that continue to be degraded by the effects of pixel fixed pattern noise. Moreover, other common techniques involve calibrating for fixed pattern noise by positioning a cap over a lens to block incident light to yield references (e.g., user placing the cap over the lens before operation); yet, these techniques oftentimes fail to account for changes in fixed pattern noise over time during operation (e.g., due to changes in temperature, gain, . . . ).